The British Prime Minister David Cameron has promised new anti-extremist measures after the UK announced it is raising its terror threat level to 'severe'.

This means an attack is deemed to be "highly likely", but there is no intelligence to suggest one is is imminent.

The British Home Seretary Theresa May has said "The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West."

"Some of those plots are likely to involve foreign fighters who have travelled there from the UK and Europe to take part in those conflicts."

She went on to add "We face a real and serious threat in the UK from international terrorism. I would urge the public to remain vigilant and to report any suspicious activity to the police."

The decision comes amid growing concern about hundreds of aspiring British jihadis travelling to Iraq and Syria, where Islamic State militants have seized large swathes of territory, and the murder of American journalist James Foley.

National Policing Lead for Counter-Terrorism, Assistant Commissioner Mark Rowley, said: "We need communities and families to bring to our attention anyone they perceive may be vulnerable, a danger or escalating towards terrorism."

"From this afternoon we will begin to increase our levels of visible patrols and implement other security and protection measures. We will also build on existing community relations to provide reassurance and seek their support and assistance in keeping the UK safe."

'Severe' is the second-highest terror threat level in Britain - after 'substantial' (an attack is a strong possibility) but before critical (an attack is expected imminently).

Mr Cameron says there will be new legislation to make it easier to take passports away from people who would travel abroad to fight.

While Ms May insists there is no imminent threat of an attack.